The vagus nerve is a key component of the afferent pathway between the gut and the brain. Vagal afferent discharge is stimulated by the macronutrient content of the gut lumen resulting in reflex changes in gastrointestinal function and inhibition of food intake. In the previous funding cycle, significant progress was made in understanding the mechanisms by which lipid (long chain triglycerides) and protein activate vagal afferents (sensory transduction). Both lipid and protein stimulate the vagal afferent pathway via release of CCK and CCK binding to CCK1 receptors on vagal afferent nerve terminals in the intestinal mucosa. Increases in vagal afferent fiber discharge by lipid depend on the formation of chylomicrons and release of apolipoprotein A-IV, release of CCK and stimulation of CCK1 receptors on vagal afferent terminals in the duodenal mucosa. The ability of protein to stimulate the CCK1R and vagal afferent pathway is dependent on expression of the proton coupled oligopeptide transporter, PepT1, in the intestinal mucosa. Regulation of food intake and postprandial gastrointestinal function is altered by long-term changes in dietary composition, such as a high fat or a high protein diet. The focus of the current proposal is to determine the alterations that occur in the vagal afferent pathway during long-term changes in macronutrient content of the diet. Two hypotheses will be tested. The first hypothesis is that the sensitivity to long chain triglycerides is decreased by chronic high fat diet due to decreased apo A-IV secretion. Maintenance on a high fat diet, to decrease intestinal apo A-IV secretion, and apo A-IV deficient mice will be used in experiments to determine the alterations in lipid-induced inhibition of gastric emptying, stimulation of vagal afferent fiber discharge, activation of second order neurons in the brainstem, release of CCK and expression of CCK1 receptors by vagal afferents. The second hypothesis to be tested is that long-term ingestion of a high protein diet increases the sensitivity of the sensory transduction for protein;this increase is due to increased expression of the oligopeptide transporter, PepT1. This hypothesis will be tested by maintenance of rats on a high protein diet and determining changes in vagal afferent activation by protein and CCK, expression of CCK1 receptors, release of CCK and intestinal feedback regulation of gastric function by protein. The increasing prevalence of obesity is a major health problem in the Western world, because it is strongly associated with several chronic diseases. The significance of the proposal is that a greater understanding of these neural pathways and their modification by diet will provide opportunities for the rational design of new treatments for a number of diseases including irritable bowel disease and obesity.